Tang getting nipped any help appreciated


Reefing newb
I have five fish in my 54 gallon tank. Tang, dotty back, 2 clowns and a damsel. Fish only no coral

At first I thought it had fin rot so treated the tank for 7 days then did a water change. Fins look they are getting worse. Is there anything I can do? Heard that the dotty back could be the culprit?